The present invention relates to a fuel cell assembly comprising a stack of a plurality of planar fuel cells, and is particularly concerned with such a fuel cell assembly in which the compressive load on each fuel cell is independent of its position in the stack. The invention also extends to a single fuel cell assembly.
A fuel cell assembly comprising a stack of a plurality of planar fuel cells requires interconnect means between each pair of adjacent fuel cells to transfer electrical current and heat from the fuel cells, to facilitate the conveyance of oxygen-containing gas and fuel gas to respective sides of each fuel cell, and to keep the oxygen-containing gas and fuel gas apart.
In a single fuel cell planar fuel cell assembly the interconnect means are effectively terminal plates which transfer electrical current and heat from the fuel cell and facilitate the conveyance of oxygen-containing gas and fuel gas to respective sides of the fuel cell. Likewise, the end interconnect means in a stack of planar fuel cells are effectively terminal plates. However, for convenience, all the aforementioned interconnect means, whether between adjacent fuel cells or terminal plates, will hereinafter be referred to as xe2x80x9cinterconnect membersxe2x80x9d.
Commonly, the fuel cells and interconnect members have the same cross-sectional area and the electrical contact between the cells and interconnect members and the sealing of respective sides of each fuel cell from each other is maintained by using the force imparted by the weight of the cell(s) and/or interconnect members above any one cell. Thus, the fuel cells are fully load bearing. Examples of such an arrangement are described in international patent applications PCT/AU96/00140 and PCT/AU96/00594. The problem with this approach is that the lower cells in the stack carry greater weight than the upper cells. For a stack with a large number of fuel cells the load on the lower cells can be significant.
The carrying load of a ceramic, such as in a solid oxide electrolyte fuel cell, is much higher in compression than in tension and the fully load-bearing arrangement described above assumes that, in a perfect system, the cells carry only a compressive load. This compression-only model acquires near perfect flatness of all load carrying parts since unevenness will lead to tensile forces in the structure and to the possible breakage of the fuel cells. In practice it is not possible to ensure such a quality of flatness in all of the load carrying parts.
The problem of possible breakage of solid oxide electrolyte fuel cells due to tensile forces applied to them in use has not been very substantial in the past due to the relatively high strength structure of previously proposed fuel cells such as those described in the aforementioned international patent applications. In this type of fuel cell, the solid oxide electrolyte layer is relatively thick compared to the anode and cathode layers applied to respective sides and has substantial strength. However, recent developments have introduced solid oxide electrolyte fuel cells in which the electrolyte layer is considerably thinner and is not a primary load bearing layer. Instead the porous anode layer acts as the primary load bearing layer and uneven or excessive loads applied to these fuel cells can be very destructive.
A further problem with a fuel cell in a stack carrying the full mass of the fuel cells and interconnects above it is that the relatively weak porous electrode layers of the fuel cell may collapse under the load.
European patent application EP 0568991 describes a fuel cell assembly comprising a stack of a plurality of planar fuel cell structures, each comprising a fuel cell and a single interconnect member on one side. Each fuel cell structure is located in a hollow plate and is separated from an adjacent fuel cell structure by a hollow intermediate plate, with the interconnect member of one fuel cell structure being maintained in electrical contact with the anode of an adjacent fuel cell structure by a felt-like nickel metal conductive material disposed in the hollow intermediate plate. Each fuel cell carries the load of the associated interconnect member. Furthermore, each fuel cell structure is compressed between a seal element and inlet defining elements of the adjacent intermediate plates so that increasing compressive loads may still be applied to fuel cells down the stack.
It is an object of the present invention to alleviate the aforementioned disadvantages of the prior art.
According to the present invention there is provided a fuel cell assembly comprising a stack of a plurality of planar fuel cells each comprising an electrolyte layer having an anode layer on one side and a cathode layer on the other side and a plurality of interconnect members, each fuel cell being disposed between and in electrical contact with an adjacent pair of interconnect members with oxygen-containing gas passage means being formed between the cathode layer of each fuel cell and the adjacent interconnect member and fuel gas passage means being formed between the anode layer of each fuel cell and the adjacent interconnect member, and wherein a chamber of greater height than the thickness of the respective fuel cell is defined between the adjacent interconnect members in each pair within which the fuel cell is received, and electrically conductive compressible means also disposed within the chamber in electrical contact with a first side of the fuel cell and the adjacent interconnect member urges the fuel cell towards the adjacent interconnect member on the second side thereof to maintain the fuel cell in electrical contact with both adjacent interconnect members.
By this arrangement, each fuel cell is displaceably received within the respective chamber but for the associated electrically conductive compressible means and the compressive load on each fuel cell is provided by the respective compressible means. Thus, the compressive load on each fuel cell is independent of the position of the fuel cell in the stack. This means that a reduced compressive load may be applied to each fuel cell which is particularly advantageous for the aforementioned solid oxide electrolyte fuel cells in which the electrolyte layer is not a primary load bearing layer. It also means that the load conditions in each chamber can be the same throughout the stack, so that the properties of the materials used in the stack do not need to vary according to the position of the fuel cell in the stack.
The invention is also applicable to a fuel cell assembly comprising a single fuel cell. Accordingly, the invention further provides a fuel cell assembly comprising a planar fuel cell having an electrolyte layer with an anode layer on one side and a cathode layer on the other side, the fuel cell being disposed between and in electrical contact with respective interconnect members, oxygen-containing gas passage means being formed between the cathode layer and the adjacent interconnect member and fuel gas passage means being formed between the anode layer and the adjacent interconnect member, and wherein a chamber of greater height than the thickness of the fuel cell is defined between the interconnect members within which the fuel cell is received and electrically conductive compressible means also disposed within the chamber in electrical contact with a first side of the fuel cell and the adjacent interconnect member urges the fuel cell towards the adjacent interconnect member on the second side thereof to maintain the fuel cell in electrical contact with both interconnect members.
The compressible means may take any of a variety of forms which maintain at least a minimum desired compressive force on the fuel cell even at the operating temperature of the fuel cell assembly. It is desirable that the compressible means maintains electrical contact between the fuel cell and the interconnect member during the full life of the fuel cell in use and therefore that it is not subject to more than minimal creep under the compression load. Any creep should not be so great as to cause the electrical contact to be broken.
Preferably the compressible means is disposed on the anode side of the fuel cell. Examples of the metal or metallic material which may be used for or in a compressible means on the anode side of the fuel cell include nickel, nickel alloy such as nickel-chrome and nickel-aluminium, and oxide dispersion strengthened nickel. Alternatively, the nickel may be replaced by other suitable metal or metals from Groups 8-11 of the Periodic Table.
Possible examples of the compressible means for use on the anode side of the fuel cell include a structure, such as a metallic corrugation or a porous metallic felt, which retains some resilience at the operating temperature; and a composite of a porous brittle material and a metal.
The composite of brittle material, such as a ceramic, and a metal may be designed such that the brittle material yields at, the applied loading but does not fail completely so that it will maintain pressure on the fuel cell between upper and lower limits. The metal maintains the electrical path, and preferably a heat path, through the composite and may provide reinforcement for the brittle material.
Preferably the compressible means is resilient so that a compressive load may be maintained during, for example, temperature cycling of the fuel cell assembly. In a preferred embodiment, the resistance is provided by a corrugated metal or metallic sheet, optionally with a substantially flat sheet of metal or metallic material disposed between the corrugated sheet and the first side of the fuel cell to alleviate any sliding movement between the corrugated sheet and the first side of the fuel cell as the corrugated sheet is compressed.
The flat sheet must permit the gas to contact the first side of the fuel cell and may be porous or otherwise have gas flow passages therethrough. Preferably, the flat sheet is formed of expanded material, that is with an array of slits formed in the material and the sheet being stretched to open up the slits.
If the gas passage means on the first side of the fuel cell is formed between the adjacent interconnect member and the corrugated sheet, the corrugated sheet must be porous or otherwise have gas flow passages therethrough. In the preferred embodiment, the corrugated sheet is formed of expanded material. However, the corrugated sheet may be used to define the gas passage means, in which case it may not need to have gas flow passages therethrough.
The corrugated sheet may be joined to the adjacent interconnect member, for example by spot welding, in order to control the compressibility of the corrugated sheet. Alternatively, the substantially flat sheet of metal or metallic material may be disposed between the corrugated sheet and the adjacent interconnect member and may be joined to the corrugated sheet by, for example, spot welding. This substantially flat sheet may also be of expanded material or otherwise have gas flow passages therethrough and may be joined to the interconnect member, for example by spot welding.
The compressible layer may alternatively comprise an integral compliant layer on the interconnect member which is adjacent the first side of the fuel cell.
For a compressible means on the cathode side of the fuel cell, in addition to having electrical conductivity and porosity the material should be resistant to oxidation, for example a form of ceramic felt or other fibre structure.
Electrically conductive compressible means as described above may also be disposed within the chamber in electrical contact with the second side of the fuel cell and the adjacent interconnect member. Advantageously, compressible means on only one side is disposed on the upper side of the fuel cell.
The chamber for the fuel cell may be defined by a recess in one or both of the adjacent interconnect members, with the interconnect members being electrically insulated from each other around the chamber, and/or by an insulating spacer between the adjacent interconnect members around the chamber. The insulating spacer may be formed of an insulating material, for example a ceramic such as alumina, or, for example, of a conductive material, such as a metal, having an insulating coating or surface layer thereon.
Gas flow channels across one or both sides of the interconnect members for flow of oxygen-containing gas and/or fuel gas to respective sides of the fuel cells may be formed in the interconnect members. To minimise machining or other finishing of the interconnect members, the channels for gas flow to at least the first side of the fuel cell are advantageously defined by the compressible means, as described above, and the compressible means advantageously contacts a flat side of the adjacent interconnect member. The interconnect member may conveniently be formed of a stainless steel.
The fuel cell assembly may be externally manifolded for the gas flows, for example as described in the aforementioned EP 0568991, but is preferably internally manifolded. Thus, the oxygen-containing gas and fuel gas supply and exhaust passages preferably pass through the interconnect members, and possibly through the aforementioned insulating spacer if provided.
A seal which is advantageously compressible in use is conveniently provided around a peripheral portion of the fuel cell between the second side thereof and the adjacent interconnect member to seal the fuel gas and the oxygen-containing gas in the chamber from each other. Since the electrode layers may have a degree of porosity, it is desirable for the electrode layer on the second side of the fuel cell, preferably the cathode layer as described above, to not extend into the peripheral portion of the fuel cell, so that the seal engages the electrolyte layer. Preferably, when the compressible means is provided on only the first side of the fuel cell, the seal is compressible in use to the extent that the electrode on the second side of the fuel cell abuts and makes electrical contact with the adjacent interconnect member at the operating temperature. In a preferred embodiment, the seal is solid at room temperature, and therefore during assembly, but becomes viscous at the operating temperature, for example 700xc2x0 C. to 1,000xc2x0 C., of the fuel cell assembly. Advantageously, the seal is a glass-containing gasket which may comprise plural layers of glass containing material.
The seal, or a separate seal member, may extend between the adjacent interconnect members or between the interconnect member adjacent the second side of the fuel cell and the aforementioned insulating spacer. A further seal member, which may be thinner than the seal and/or the first-mentioned seal member may be provided between the insulating spacer and the interconnect member adjacent the first side of the fuel cell. The first and second mentioned seal members may be formed of the same or similar material to the seal and may also be compressed in use of the fuel cell assembly.
The or each fuel cell may be one of an array of fuel cells in a respective layer of plural planar fuel cells in the assembly, with each fuel cell being disposed in a chamber in accordance with the present invention. Advantageously, the interconnect members adjacent a common side of all or more than one of the fuel cells in each array are formed in a single plate. Likewise, the aforementioned insulating spacer, if provided, may define partly or wholly all or more than one of the chambers for the fuel cells in each array.